1. Field of the Invention
The invention relates to a data driver used in a current-driving display device, and more particularly, to a current-storing/reproducing data driver including a current storing/reproducing module.
2. Description of the Prior Art
n OLED (Organic Light Emitting Device, OLED) display can be designed as a thin, flat panel display device. The OLED display can be found in a plethora of electronic goods, ranging from notebook computers and digital cameras, to flight avionics and medical diagnostic tools. OLEDs offer crisp, high-resolution images, and have the primary advantage of offering relatively low power-consumption rates while still maintaining good color contrast and screen refresh rates. The OLED is an electrically driven lighting element having a brightness that depends on the magnitude of a related current. At present, the magnitude of the brightness (which is also called the gray-scale value) is controlled by the magnitude of the OLED driving current in an application OLED matrix display.
Base upon the driving method, the matrix display can be classified as either a passive matrix or an active matrix display. Passive matrix displays adopt the method of driving the scan lines of the display in sequence, driving pixels in different rows sequentially. Since the light-emitting time of each pixel is restricted by the scanning frequency and the numbers of scan lines, the passive matrix method is not suitable for large-sized and high dots-per-inch (dpi) displays. Active matrix displays, however, possess an independent pixel circuit for each pixel, which is described in FIG. 1, which is a schematic diagram of a pixel 20. The present embodiment of the pixel 20 includes a capacitor C1, an OLED D, and a plurality of MOS transistors or TFTs(Thin-film Transistors) T1-T4. With this arrangement, even in large-sized and high dpi displays, a steady driving current I is provided for each pixel, which improves the brightness balance.
For achieving advantages of power saving, integrity, and cost effectiveness, more OLED systems adopt the digital type as an input data type so that the digital-to-analog converter should be involved in the data driver. In addition, the brightness of the OLED display is controlled by current. Therefore, the digital-to-analog process should be achieved by a digital-to-analog current converting circuit to convert digital data into an analog current signal. The corresponding pixel is also a current-driving pixel as the pixel 20 shows in FIG. 1. Please refer to FIG. 2, which is a functional block diagram of a prior-art data driver 10. The data driver 10 corresponds to the pixel 20 of a display device as shown in FIG. 1. The data driver 10 includes a level shifter 12, a latch 14, a shift register 16, and a digital-to-analog current converter 18. The level shifter 12 is used to adjust the potential levels of a received digital signal (a 6-bit digital signal), and the latch 14 is electrically connected to the level shifter 12 for storing and buffering the digital signal. The latch 14 can temporarily store the 6-bit the digital signal so that the latch 14 is a 6-bit latch. The shift register 16 can be used to generate a shift-register signal to transmit the digital signal to the level shifter 12 at one time. Afterwards, the level shifter 12 will execute the potential-level adjusting and buffering functions and transmit the digital signal to the latch 14. The digital-to-analog current converter 18 is connected to the latch 14 for receiving the digital signal outputted from the latch 14. The digital-to-analog current converter 18 can be used to transform the digital data into an analog current signal and to output the analog current signal to a data line 19. According to the amplitude of the analog current signal, the gray colors of the display panel can be determined.
Taking a display panel with 4-bit input digital data as an example, J. Kanicki et.al. (U. of Michigan, USA) has disclosed a simple digital-to-analog current converter installed with a set of TFTs (Thin Film Transistors) with a width-to-length ratio assigned as 1:2:4:8 and a current source to generate 16 current gray scales, I. Please refer to FIG. 3, which is a schematic diagram of an embodiment of a prior-art digital-to-analog current converter 18. The digital-to-analog current converter 18 is composed of a plurality of transistors T5-T9. Due to that the 16 current gray scales rely on 4 (1:2:4:8) TFTs T6-T9, any fluctuation of threshold potential level and mobility in each TFT will generate significant variation to affect the current gray scales. Furthermore, the quality of the corresponding panel will be influenced. In addition, because the output impedance of the digital-to-analog current converter 18 is not high enough, the output potential level will be affected by a current flow passing the digital-to-analog current converter 18. Therefore, when the digital-to-analog current converter 18 is connected to the corresponding pixel, the output current may not be a stable 16 gray-scale current.